Delivery of neurotrophic factors to sites of spinal cord injury (SCI) has been shown to induce axonal growth from different neuronal populations. While cellular growth factor gene delivery is a potent means of promoting the growth of several axonal populations into spinal cord lesion sites, novel strategies need to be developed to increase the distance of axonal growth and to define the conditions needed to direct axonal growth across a lesion site. In studies supported by an R21 grant we have determined that in vivo neurotrophin gene transfer beyond a spinal cord lesion site establishes gradients of neurotrophic factors and can thereby direct axons across a spinal cord lesion site. The mechanisms underlying morphological and functional outcomes and the conditions needed to achieve persistent morphological and functional improvements will be studied in this proposal. We will investigate the hypothesis that transient, regulated gradients of the neurotrophin BDNF will be sufficient to induce axonal bridging across a spinal cord lesion site, morphological plasticity and reinnervation, resulting in persistent functional recovery. BDNF gradients across a cervical spinal cord lesion site filled with bone marrow stromal cells will be established using in vivo lentiviral gene transfer. Mechanisms of morphological and functional outcomes will be investigated in detail in well-defined transection models of SCI that allow the discrimination between sprouting of spared projections and true regeneration of injured axons. Anterograde and retrograde tracing and specific re-transections will allow the identification of different mechanisms that could contribute to reinnervation and functional recovery. Using tetracycline-regulated, lentiviral BDNF gene transfer we will define the conditions needed to retain the connectivity and function of regenerated axons and we will determine whether a practical need for extended gene expression exists for the formation and maintenance of newly formed synapses. If successful, these studies will establish practical strategies for growth factor delivery to the injured spinal cord that can lead to the development of novel therapies for spinal cord injury.